Complementary metal oxide semiconductors, also sometimes called complementary-symmetry metal oxide semiconductors or CMOS, is a known class of integrated circuits (ICs). CMOS technology is used in, for example, microprocessors, microcontrollers, static RAM, and other digital logic circuits, as well as analog logic circuits. Typical digital CMOS designs implement complementary and symmetrical pairs of p-type and n-type metal oxide semiconductor field effect transistors (MOSFETs) for logic functions. The physical structure of certain field-effect transistors in CMOS typically includes a metal gate electrode placed on top of an oxide insulator which, in turn, is on top of a semiconductor material. Some current gate electrodes are made from a material other than metal such as, for example, polysilicon, but the term CMOS often is still used to refer to these designs.
CMOS devices typically have high noise immunity and low static power consumption. Generally, significant power is drawn only when the transistors in the CMOS device switch between on and off states. Thus, CMOS devices generally do not produce as much heat waste as other forms of logic. CMOS technology also allows for a high density of logic functions on a chip. Indeed, technology scaling in CMOS devices advantageously has led to thinner gate oxides, more shallow junctions, higher doping densities, etc.
Unfortunately, however, the same technology scaling also has had a negative impact on electrostatic discharge (ESD) immunity. Technology scaling in CMOS increases the density because the digital circuit can be scaled. However, the ESD circuit cannot be scaled, and thus a corresponding ESD immunity cannot be achieved when the technology is scaled. ESD generally refers to the movement of static electricity from a nonconductive surface, which can damage semiconductors and other circuit components in integrated circuits.
ESD may be imparted to an integrated circuit within an IC package when the integrated circuit or the IC package is contacted by an electrostatic charge source that may be encountered during assembly, or afterwards, e.g., when the device is in the field. For perspective, it is noted that a person walking on a carpet sometimes can carry an electrostatic charge of up to several thousand volts under high humidity conditions and over 10,000 volts under low humidity conditions.
Thus, it will be appreciated that there is a need in the art for techniques that improve the ESD immunity of CMOS ICs. It also will be appreciated that there is a need in the art for techniques that maintain and/or improve the ESD immunity of an advanced CMOS IC when the technology is scaled.